A balanced flue is a double walled. Fresh air comes in one direction from outside and the combustion air from burning the gas is drawn back outside. Depending on the fire, you can often put the flue straight out the back and terminate on an external wall so there is no need to have a chimney going up through or outside your property. Conventional flue uses an existing chimney to make the gas fire or stove work. The chimney will often need to be lined with a gas stainless steel flexible liner.